We have continued to investigate this property of insulin receptors known as negative cooperativity. The affinity of insulin receptors decreases with occupancy, due to site-site interactions which accelerate the dissociation of the insulin-receptor complex. We have done extensive modeling and computer analysis of this system. A simple model has been proposed which predicts the properties of the insulin receptor and provides insights into its minimal structure. The model was also used successfully in computer curve fitting of experimental data. We have also demonstrated that microfilament modifying drugs decrease the concentration of available insulin and growth hormone receptor sites on IM-9 lymphocytes, whereas microtubule modifying drugs are ineffective. In neither case was the negative cooperativity altered.